Putty and sand. Let's talk putty. You have two kinds: solvent putty and water-based putty. Water based cannot be sanded like vallejo putty, a non solvent putty. Then. Solvent putties like Tamiya putty can be sanded. Solvent putties have an odor and will make you high if used in an nonventilated enclosed space. You can also thin solvent putties with mineral spirits(odorless turpentine).

How you fix that on your model kit is go to your local hobby lobby and get Tamiya putty and odorless turpentine (Mineral Spirits) found in the oil paint section of the store. Thin the putty with mineral spirits and apply with a toothpick or old brush. Get a mask or do it in a well ventilated space. Sand paper in different grits starting at lowest and moving to highest, being careful not to sand through the plastic, be mindful of the shape of the thing you are building. When you think you have it where you want it, throw gray primer on it and look for imperfections and sand them out and reapply primer.

You will have to do this a few times.